The present invention relates to papermaking and refining of lignocellulosic and other natural and synthetic fibrous materials in the manufacture of paper, paperboard, and fiberboard products. In particular, the invention relates to replacable refiner fillings used in the process of refining chip or pulp.
In nearly all production refining equipment in use today including beaters, jordans, conical refiners, multi-disc, and disc refiners, the refining working surfaces of the refiner fillings are comprised of closely spaced bars and grooves which work against each other through relative rotation while the fibrous material passes between them. The clearance between the opposed bar and groove working surfaces determines the power applied to the refiner, as well as the extent of refining of the fibrous material.
In each kind of refiner equipment, it is often desirable to make bars as narrow and as closely spaced from each other as possible in order to achieve maximum bar edge length for the refiner with resultant distribution of the refiner power over a greater number of bar contact or bar crossing points. This relative intensity, or specific edge load as it is called, is widely recognized as an important quality parameter for most paper and board products.
While the bars of any refiner type can be of any practical width and spacing, the actual width and spacing are limited by the materials and methods used to make them, or by the cost to make them, or both. In a typical disc refiner, the replacable working surfaces, or refiner plates as they are most commonly called, may be made by casting or machining. In some instances they may be made by fabricating wherein appropriately spaced bars are affixed by welding onto a base.
In the case of cast refiner plates, the width of the bar and the width of the groove are limited to no less than about 1/8". At normal groove depths of 1/4" or so, cast bars narrower than this are prone to breakage due to internal flaws, and the need to have a draft angle of 3 deg. or so for the casting process, causes the groove volume (which provides for passage of fibrous material) to be greatly diminished at closer bar spacing than about 1/8".
In the case of machined refiner plates, the limiting factor is cost. The cost is more or less proportional to the number of grooves which must be milled to the required depth in a solid steel blank.
In the case of fabricated plates, cost is also a constraint because bars are individually welded.
Another important feature of replacement refiner plates is their useful life. During operation, the bars become worn down, until at some point, the depth of the groove between bars is so shallow that the refiner can no longer adequately transport fibrous material through the refiner plates. There are several causes of wear including abrasive nature of the fibrous materials and other particles in the medium, and the clashing of the refiner plates in the event of sudden interruption of the flow of process material.
The precise nature of the wearing of refiner plates is not fully understood. Hardness of the bar material has been shown to be an important factor. It has also been demonstrated that the rate of wear is very closely related to the corrosion resistance of the bar material.
In general, a compromise must be reached between the hardness, corrosion resistance, and toughness of the material that is chosen for a cast or machined refiner plate. Toughness is a required property because occasional tramp metal contamination occurs in the process medium. If the plates were to shatter when a piece of metal passed through the refiner, it would cause severe and costly operational problems for the paper or board mill.
There are several potential wear advantages to fabricated or machined refiner plates, however a serious limitation results from the necessity of producing refiner discs in a complete circle configuration. A full circle replacement plate for a 34" or larger refiner will weigh several hundred pounds thus requiring lifting aids for installation into, and removal from, a refiner. Cast refiner plates can be, and usually are produced in segments, with each segment being 30, 45, or 60 degrees and with 12, 8, or 6 segments respectively being required to make up a complete replacement working surface for a single disc of a disc refiner. Each segment will weigh less than 35 pounds, and will usually be individually bolted into the place on the disc, such that an entire set of plates can be replaced by a person without the need for special lifting devices. For this and other reasons, most replacement disc refiner plates are castings, usually of special cast iron or stainless steel alloys.
As a practical matter, one of the reasons machined or fabricated plates are not produced as segments has to do with an operational requirement for non-parallel edge crossing of the refiner bars for processing fibrous material. If a stator plate and a rotor plate, whose working surfaces act against each other, contain bars whose leading edges pass each other in parallel or nearly parallel condition, there is a known tendency for excessive cutting of the fibrous material being processed. Thus it is often a process requirement that a refiner plate does not have any precisely radial bars, but rather that it have bars with at least a slight offset or oblique from a radial orientation, typically between 3 and 20 degrees.
Refiner disc plate segments have precisely radial side edges such that it is a somewhat costly complication to produce a disc working surface pattern having no precisely radial bar or groove at the segmental dividing lines. Therefore, the segment joint must cut across the pattern of bars and grooves at a shallow angle. This requirement is difficult and costly to accomplish in the case of machined and fabricated plates and which, even in the case of cast plates, leaves narrowly tapered bars likely to be very much weakened at their extremities.
In sum, the utility of disc refiner plates is limited by the operational requirement for bars oriented obliquely to radial, by consequent manufacturing limitations, and by the rate of working surface wear through corrosion and abrasion.